EPIBLAST, MESOBLAST, AND HYPOBLAST. 211 



henceforward exhibit difterent tendencies from those 

 which are possessed by the rest of the blastoderm. In 

 fact, it is the primitive alimentary apparatus or archen- 

 teron, and its wall is termed the hypoblast. The rest of 

 the blastoderm, on the contrary, is the primitive epider- 

 mis, and receives the name of epiblast. If the food- 

 yelk were away, and the archenteron enlarged until the 

 hypoblast came in contact with the epiblast, the entire 

 body would be a double-walled sac, containing an ali- 

 mentary cavity, with a single external aperture. This is 

 the gastrula condition of the embryo ; and some animals, 

 such as the common fresh- water polype, are little more 

 than permanent gastrulce. 



Although the gastrula has not the slightest resem- 

 blance to a crayfish, yet, as soon as the hypoblast and 

 the epiblast are thus differentiated, the foundations of 

 some of the most important sj^stems of organs of the 

 future crustacean are laid. The hypoblast will give rise 

 to the epithelial lining of the mid-gut ; the epiblast 

 (which answers to the ectoderm in the adult) to the 

 epithelia of the fore-gut and hind-gut, to the epidermis, 

 and to the central nervous system. 



The mesodermal structures, that is to say the con- 

 nective tissue, the muscles, the heai-t and vessels, and 

 the reproductive organs, which lie between the ectoderm 

 and the endoderm, are not derived directly from either 

 the epiblast or the hypoblast, but have a g^itasi-independent 

 origin, from a mass of cells which first makes its appear- 



